An in vitro investigation of plasma sprayed hydroxyapatite (HA) coatings produced with flame-spheroidized feedstock

The in vitro behaviour and characteristics of plasma sprayed hydroxyapatite (HA) coatings using flame-spheroidized HA feedstock powder on titanium alloy (Ti-6Al-4V) substrates were investigated in a simulated physiological environment as an attempt to reflect the actual incubational condition of an implant in a human body system. As-sprayed and heat-treated HA coatings were immersed in a simulated body fluid with ionic concentrations comparable to that of human blood plasma for time intervals 2, 4, 6, 8 and 10 weeks. Rapid dissolution of calcium phosphate was found to occur within the first 4 weeks, and after the 5th week a retarding rate of 4.1mM week^-1 was observed where precipitation, nucleation, and, growth of a carbonate-containing, poorly crystallized or amorphous calcium phosphate layer on the as-sprayed coatings were noted. The heat-treated coatings showed minimal or no precipitation on the surface except for the presence of calcite minerals that is due to carbonation effect. Complete dissolution of other calcium phosphate phases such as tetracalcium phosphate, tricalcium phosphate and calcium oxide was also noted after 2 weeks of immersion due to higher ionic solubility relative to HA. A declining trend in respective microhardness and elastic modulus of the as-sprayed HA coatings from 207.06±3.2H_k300 to 131.8±5.2H_k300 and from 31.37±1.4 to 19.81±1.6 GPa was observed after 10 weeks of immersion. Tensile bond strength of both types of coatings showed similar declining trend, with an average dip from 24.5±2.4 to 7.9±2.6 MPa. Nevertheless, the heat-treated samples showed rather reasonable mechanical stability and structural integrity of 26.7±1.4 GPa in elastic modulus after soaking.